The present invention relates to a grid assembly for ion beam sources and method therefor and, more particularly, to integrated single- and multi-grid assemblies and methods therefor for use in ion beam systems that are used for various surface treatments including etching and material deposition.
Ion beams are used in various processes including the reactive and non-reactive deposition of materials onto substrates, etching of metals, semiconductors, and insulating materials, and the surface treatment of materials. In ion beam deposition, the ion beam strikes a target to release atoms which then travel to substrates where the atoms nucleate. This type of operation often requires a focused ion beam of sufficient energy in order to release target atoms at a practical rate.
Whether the ion beams are used for etching or for deposition, the ion beam source typically includes one or more grids that provide a desired beam profile and direction. The use of these grids allows the production and focussing of ion beams of different energies so that the ion beam can be tailored to the desired application. A typical ion source is mounted in a vacuum chamber or other low pressure environment for sputtering atomic particles from a target onto a substrate. The ion source includes a main body that contains an electrically heated cathode, one or more anode elements, and a magnetically constrained plasma chamber into which a working gas, such as argon, is introduced. The exit end of the ion source includes at least one multi-apertured grid through which ions pass and, in many cases, a two-grid set having a first screen grid and a second accelerator grid. Lastly, a neutralizer wire can extend across a diameter line at the exit end of the ion source. Each of the elements of the ion source, the cathode, the anode, the grid(s), and the neutralizer are maintained at specific electrical potentials to create ions and form those ions into beamlets which combine to form the ion beam.
The most common form of ion optics utilizes a pair of multi-apertured grids: a screen grid adjacent the discharge plasma and an accelerator grid immediately adjacent the screen grid. The extraction grids are usually fabricated from thin sheets of stainless steel, graphite, or molybdenum into which a multitude of apertures are mechanically drilled in a particular pattern and aperture spacing (usually less than 1 mm). While this method of fabrication is generally satisfactory, the mechanical machining can induce substantial internal stresses in the finished grid; these stresses tend to cause undesired distortions when the grids are heated during use. Additionally, the mechanical machining also causes the formation of flakes or chips that can adhere to the grids and adversely affect the beam profile.
The apertures of both the screen and aperture grids are accurately co-aligned, usually by providing alignment pins on the exit end of the ion source and alignment bores in the grid assembly itself so that the grids can be slid onto their mounting pins to assure and maintain a reasonable co-alignment of the various apertures of the screen and accelerator grids. If the apertures of the screen and accelerator grids are accurately co-aligned, the maximum ion-beam current will be obtained with minimum exposure of the material of the grids to ion impingement. If the grid apertures are misaligned, the ions can directly impinge the material of the accelerator grid, thereby lowering the ion beam current, changing the ion beam direction and the trajectory of individual ions, cause undesired variations in the beam profile, erosion of the aperture edges, and possible contamination of the target with accelerator-grid material.
In general, grid misalignment can result from sub-optimal assembly in the field after routine maintenance of the beam source, mis-handling of the grid assembly, over-temperature operation, and changes in the physical dimensions of the grids as a consequence of continued thermal cycling.
Efforts have been made to overcome the problems attendant to the mechanical machining of the grid materials by using semiconductor wafers as a starting material and microelectronic fabrication techniques as disclosed in J. L. Speidell, J. M. E. Harper, J. J. Cuomo, A. W. Kleinsasser, H. R. Kaufman, and A. H. Tuttle "The Fabrication and Use of Silicon and Gallium Arsenide Ion Source Extraction Grids" J. Vac. Sci. Technol., 21(3), Sep./Oct. 1982 pps. 824-827. As disclosed therein, single crystal silicon wafers with a (100) orientation are patterned with an array of rectangular openings to define the grid area and etched to define inverted truncated pyramids bounded by four (111) planes intersecting with the two (100) surfaces of the wafer. Etched silicon grids perform well in comparison to conventional graphite grids, although the problem of aligning the screen and accelerator grids nonetheless remains.